This invention is directed to a method of producing twisted, curly fibers from wet wood pulp.
Wood pulp is commonly used to make paper as well as absorbent articles. When wood pulp fibers are flat, the fibers lack absorbency and softness compared to wood pulp fibers that are twisted or curly.
In the past, curling of fibers has been done primarily by mechanical means, resulting in densification of portions of the fiber wall and mechanical damage to fibers. Also in the past, many cross-linking efforts have tended to decrease the swellability of fibers.
Never-been-dried wood pulp has many fine pores within the cell walls in a multi-lamellar fashion. The pores are commonly referred to as intra-fiber capillaries, in contrast to inter-fiber capillaries that are formed between individual fibers. The intra-fiber capillaries of a never-been-dried pulp are highly vulnerable to outside forces such as the surface tension of water, electrolytes, mechanical and thermal treatments to name a few. In particular, intra-fiber capillaries are easily collapsed during conventional thermal drying, such as during drum drying. When the intra-fiber capillaries of a never-been-dried pulp collapse during drying, the width, or diameter, of individual fibers shrinks. As a result, the morphology of once-dried wood pulp tends to be flat and ribbon-like, and the intra-fiber capillaries practically disappear.
Once-dried fibers can be re-wet to open up and increase the swellability. If a fiber does not shrink uniformly during drying, its fiber morphology will be quite different from the conventional ribbon-like fiber morphology. Such fibers that shrink non-uniformly are likely to be coiled or twisted. The degree of coils or twists per individual fiber depends on the number of intra-fiber capillaries within the wood pulp and the degree of non-uniform shrinkage of fiber diameters along their fiber axes, i.e., perpendicular to the fiber diameter direction.
In order to obtain a short drying time during a thermal drying process such as flash drying, wet pulp is conventionally defiberized into low density, individual fibers prior to drying so that the largest possible pulp surface is exposed to the hot drying air. Such defiberization is known as wet fluffing. It is believed by many that the fluffing operation is the key to a successful flash drying system. Unfortunately, however, a thorough wet fluffing is difficult to achieve, generally requiring multiple steps. For example, one particular fluffing method treats moist cellulosic pulp fibers to a combination of mechanical impact, mechanical agitation, air agitation, and a limited amount of air drying to create fluff fibers.
Curly, twisted cellulose fibers can be produced by permanently interlocking the intra-fiber capillaries with a chemical cross-linker prior to flash drying. The use of a chemical cross-linker is unfavorable for a number of reasons. In particular, the use of a chemical cross-linker involves safety concerns since chemical cross-linkers are generally hazardous and harmful. Therefore, the use of a chemical cross-linker requires a thorough washing of un-reacted chemical cross-linker for safety. Also, the use of a chemical cross-linker is likely to cause interlocking between fibers that would be difficult to be defiberized into individual fibers for a product application. Potential damage to the fibers may occur during the defiberization stage due to interlocking of the fibers. It can be difficult to form an absorbent product due to such interlocking of fibers. Furthermore, the use of a chemical cross-linker is not very economical due to the complexity of handling such a chemical cross-linker. With respect to the present invention, such permanently interlocking intra-fiber capillary structures tend to make the fibers stiffened and destroy all the useful capillaries as fluid channels.
There is a need or desire for a method of modifying wood pulp fibers to form twisted, curly fibers without the aid of a wet fluffing process or a chemical cross-linker.